In the field of LED drivers for offline applications such as retrofit lamps, solutions are demanded to cope with requirements relating to high efficiency, high power density, long lifetime, high power factor and low cost, among other relevant features. While practically all existing solutions compromise one or the other requirement, it is essential that the proposed driver circuits properly condition the form of the mains power into the form required by the LEDs, while keeping compliance with present and future power mains regulations. It is of critical importance to guarantee a maximum with respect to perceptible light flicker (preferably zero) at the same time that the power factor is maintained above a certain limit.
Typically, two series-connected power stages are employed to obtain a high power factor while keeping the output power constant throughout a mains cycle (or supply cycle, i.e. the cycle of the mains voltage or the supply voltage). Also known are converters with a single power-converting stage that allow high power factors (HPF) by means of integrating a boost converter operating in discontinuous conduction mode. These converters actually combine two power conversion stages.
A HPF converter for compact fluorescent lamps is described in “High-Power-Factor Electronic Ballast with Constant DC-Link Voltage”, by Ricardo de Oliveira Brioschi and José Luiz F. Vieira, IEEE Transactions on Power Electronics, vol. 13, no. 6, 1998. Here, a half-bridge unit is shared by a boost converter and an LC parallel resonant converter, which is operated above resonance in order to obtain zero voltage switching (ZVS). To further support ZVS, the bus voltage is controlled to be constant. Such a HPF converter, however, typically requires a large bus capacitor and an output rectifier and has only narrow supply voltage and load (drive) voltage ranges.